The present invention relates to electrical resistance heating elements and in particular to elements having a flat configuration and consisting of a number of metal strips arranged in a meander like pattern embedded in or laminated within an insulation body.
Such heating elements are well known for U.S. Pat. Nos. 3,263,307 and 3,336,557 (both to O. G. Lund et al.) as well as from U.S. Pat. Nos. 4,025,893 and 4,092,626 (both to H. A. Bergersen), a main feature of these inventions being that the resistance metal strips are made of a material having a melting point lower than 200.degree. C. When such elements are used as electrical heating of apartments and houses and the installation necessitates close contact with combustible material such as wood and wallpaper, it is essential that the temperature rise of the heating element surroundings at no place exceeds 150.degree. C. This requirement can be achieved by using as a resistance strip a metal alloy consisting of 61.5% tin, 37.7% lead and 0.8% antimony, which has a melting point at 183.degree. C.
A heating element, where low melting resistance strips are welded between two layers of insulating material, acts as one large thermal fuse if it is operated under abnormal conditions, for example, when it is unduly covered by heat insulating materials. When the temperature in such cases approaches the melting point of the alloy (170.degree. C.+), the mechanical properties of the foil strips become very poor, and in this state, the foil may fracture at any time before the melting point is reached. At the moment the strip fracture starts, the cross-section of the strip is reduced, and it melts instantly, causing the current path to be broken. The heating element must thereafter be replaced with a new one.
There are, however, also known resistance heating elements having only one fuse. Such elements are for instance described in U.S. Pat. No 3,417,229 (G. J. Shomphe et al.), in which there is inserted a thermal fuse for each individual heater unit. Such units will only be guarded against overheating if the fuse itself is overheated. Such overheating will occur either if the fuse itself is excessively thermally insulated or if the heater unit draws excessive current. The fuse will melt at a desired temperature when overheated and cut the current so that the heater unit itself probably need not be replaced. If, however, a heater unit area somewhat displaced from the fuse is overheated, the wallpaper and surroundings may catch fire before the fuse cuts the current path.
The safest type of resistance heating elements is therefore considered to be that previously described. Experiments have shown, however, that it is not necessary for each and every square centimeter of the resistance element to be capable of fusing at the desired low temperature.